Sheet abrasive



Patented Oct. 22, 1940 UNITED STATES PATENT OFFICE SHEET ABRASIVE Francis G. Okie, Dellwood, Minn., assignor to Minnesota Mining & Manufacturing Company, St. Paul, Minn., a. corporation of Delaware 4 Claims. (Cl. 51-303) My present invention relates in general to the treatment of material, preferably in sheet form, and more particularly the treatment of material for the purpose\of forming or attaining composite structures, of which well known forms of abrasives such as what is commonly known as sandpaper is an example.

This application is a division of my copending application Serial Number 668,384, filed April 28, 1933, which is a division of my application Serial Number 338,022, filed February 6, 1929, now Patent No. 2,013,925, issued September 10, 1935. Other phases of my invention are claimed in my application Serial Number 755,445, filed November 30, 1934, now Patent No. 2,169,277, issued Aug. 15, 1939.

Although I hereinafter disclose the practice of my invention in connection with abrasives such as sandpaper, it will be understood that my invention, or certain features thereof, have a wider field of utility.

Composite structures of the abrasive type such as sandpaper or emery cloth comprise generally ('a) a base of sheet material, which may be felted as in the case of paper or woven as in the case of cloth, (b) a more or less finely comminuted resistant material having abrading properties, such as garnet or corundum, and (c) a binder generally in the form of a glue which serves to less slipperiness of the backingfthereby permitting the operator to maintain a better grip on the sheet when rubbing surfaces; the provision of an improved flexible waterproof abrasive article characterized by extensibility and compressibility to accommodate itself to bending when in use; the provision of such an article wherein the desired adhesive action between the binder and the backing is materially increased; the provision of an improved binder and abrasive article wherein a good balance of drying is attained with-.

out any tendency to crack when dry; the provision of an improved abrasive article wherein the backing or sheet is adequatelypermeated with the waterproofing medium which prevents the water or other liquid employed in wet sanding from entering the pores and eventually caus swelling of the backing, thereby maintaining the original body much longer than in any flexible abrasive article known to mej the provision of improved I coatings which tend to increase the life of the paper in use without materially decreasing the flexibility of thesame when dry; the provision of an improved binding and sizing material which has moisture proofing characteristics and is flexible when dry.

This invention in one of its forms further includes the improvement which resides in incorporating soft oils in the backing or sheet in sufficient quantity to load the sheet but in a degree insufficient to result in rendering the sheet brittle when dry, in combination with a sealing coat on the surface to increase the waterproofness of the sheet and wherein the employment of a harder binding coat when dry is compensated for by the inclusion in the sheet of a waterproofed material which maintains the flexibility of the binder and the sheet but results in the provision of a backing having greater resistance to abrasion, that is, to breaking down of the binder when employed in a wet sanding or water abrading operation.

.In the employment of my present invention for the production of a flexible abrasive article, such for example, as sandpaper and the like, suitable for employment in a wet sanding operation (which I cite for purposes of example) I employ asthe base, a sheet of material (which may be paper, cloth or the like) of such texture and thickness as will conveniently serve the employment contemplated, and as the abrasive such comminuted material (which may be crushed garnet) similarly adapted to serve the function in view, together with the binder hereinafter referred to in detail for attaching the grits to the paper; and suitable auxiliary coatings within and without the paper sheet which have characteristics which render the bond between the grit and the sheet highly tenacious and result in a relatively hard finish without materially detracting from the flexibility of the abrasive article when dry.

It is, of course, desirable that the binder utilized have the functions and characteristics not only of great mechanical strength and of great binding strength, but also that it be non-hygroscopic, permeative, moisture resistant, easily worked,

readily obtainable, relatively inexpensive and capable of facile application to the sheet.

In the manufacture of waterproof abrasive sheets, I have treated the backing or sheet with 3;;waterproofing solution and then applied coats of binder and grit, together with auxiliary coating or coatings on the back of the sheet or on both the back an'd'front. The degree of resistance to permeation by water or other liquid employed in a wet sanding operation depends in part on the amount of waterproofed materials put into the sheet, and in part upon the degree of drying to which the materials employed are subjected,

which in other flexible abrasive articles have made it necessary first to attain a fairly good drying of the auxiliary back sizing in order to give high resistance to abrasion in the backing, that is, to prevent the paper wearing off the back when the operator, is rubbing a surface in a wet sanding operation, and, though a highly efficient result is obtained in such articles, those do not have the desired high resistance to abrasion of the backing characterizing the present invention.

If, under the other methods known to me, a harder auxiliary coating material is employed, this results in stiffening the sheet beyond-the degree desired. In my present invention, I provide binding and auxiliary or sizing coats which permit of the provision of a much harder outer surface and thereby an increase in durability of the sheet, at the same time, providing for sufficient flexibility of the sheet when the grit is applied so that the cooperation of the preliminary paper treatment and sizing coats with the sheet and the bonding coat result in a balance of flexibility, together with an increase in external hardness of the sheet.

In making the new arbasive article of this invention, I prefer to apply a relatively thin top or front coat as a presizing designed not to permeate the sheet materially but serving as a penetration controlling coat when the binder coat is applied. I then apply the binder coat; then the grit. I then apply in succession two relatively soft coats to the back of the sheet, that is, to the side opposite to the grit. I prefer that the second auxiliary back sizing coat be of a softer material when set than that of the first back sizing coat so as to not materially detract from the flexibility of the finished product, sandpaper.

To sum up, after the sheet has been treated, and, therefore, includes a backing, presized on the front, that is, having at least one front presizing coat, the primary binding coat is applied to the front of the sheet, and before this coat becomes set or dry 2. layer of the grits is deposited thereon, preferably without the application of external pressure. Then I apply the backsizing coats and a sand sizing coat, that is, a coat is applied over the grits and primary binding coat for compensating for any absorption of the primary binder by the base or backing. The sheet is then, when dry, ready for use. The sheet may, for some purposes, at the outset, though not necessarily, contain soft oils tending to render the sheet itself waterproof.

From the foregoing, it will be seen that the abrasive sheet, with all of the advantages of this invention, includes the following principal elements: a treated sheet which may contain soft oils in such amount as to render the sheet substantially waterproof; a presizing front coat; a binding grit carrying coat; one or more backsizing coats which prevent the entrance of water into the back of the sheet where the operators hand comes in contact; and a sand sizing coat applied to said binding coat and grit where desirable or necessary.

It is preferable that the sand sizing coat and grit binding coat be relatively hard when dry; and that the backsizing coats be soft. It will thus be seen that a relatively hard outer surface is attained which will protect the binder coat and yet not materially detract from its flexibility and that the soft oils in the sheet when employed will permit of a flexing of the sheet without a tendency to crack. I thus attain a selection of relatively hard materials to hold the grits 0n the Sheet and relatively soft materials to give flexibility where it is most needed and to compensate for any tendency in the relatively hard binding coats to render the sheet brittle.

As an example of the relative characteristics as to hardness of the various coatings employed, the sandsize coat and binder coat preferably include, as one of their ingredients, one of the hardest oils when dry that is available for my purposes, and which does not crack when dry; a presizing designed to control penetration of these hard oils into the backing, the primary backsizing coat is of a soft material and the second coat of backsizing is of such a soft material that when placed on a glass it may be scraped off with the finger-nail several days after application.

Thus a controlled penetration of the liquid employed in an abrading operation into the pores of the flexible backing may be attained for increasing the flexibility of the abrading article in use without destroying the bond between the grit, adhesive, and sheet.

In this Way, I am enabled to provide for elongation of the backing which will prevent the finished article from becoming brittle, or cracking when bent.

In carrying out the process of manufacturing the improved abrasive article of this invention, which process I believe to be new, and in order to obtain the proper drying cycle and to reduce the time required, I prefer to apply both coats of the backsizing in the second operation, that is, for example, the coated sheet is passed through a set of squeeze rolls which apply oil on the backside and then the sealing coat is applied thereto. Instead of sending it two trips through the steps of receiving two backsizing coats, I apply both coats of backsizing in the second operation. The second coat of the backsizing thus seals the surface so that oxidation is retarded in the center of the flbre cellulose sheet.

In the present invention, I find that it is possible to employ a clear binder for retaining the grits, that is, the employment of a pigment in the binder to control penetration into the paper stock is not necessary, and I find that in the employment of a clear binder, when properly controlled, substantially all the strength of the adhesive therein may be used in retaining the grits.

The employment of a presizing coat on the sheet beneath the binder coat prevents an undue penetration of the sheet by the clear binder. I have found that I can obtain control of the application of the clear binder so as to prevent its undue penetration into the sheet by applying the binder to raw paper stock, which has been presized on the face but not previously backsized. I have found that in doing this, less penetration of the sheet by the grit binding coat takes place, probably due to the fact that in the prior operations known to me, the backsizing thinners, in evaporating, apparently exert a suction or capillary action on the binder applied to the opposite side of the sheet to hold grits and thus tend to pull the binder into the sheet.

In applying the various coatings by my pr ferred process which prevents the undue penetration of the sheet by the clear binder, I preferably first presize the paper and then apply the clear binder and a grit. The sheet thus treated and which may contain the waterproofing oils referred to, is then passed one trip through an oven maintained at a temperature of between F. to F. for 36 to 48 hours, the oven being zoned so that the sheet first gets the high heat, and the solvents employed are quickly driven off before they permeate the whole drying room. The major portion of the oven has a temperature of about 125 F. to 130 F. After this trip through the oven, the two backsizing coats and the sandsizing coat are applied, and the paper again passed one trip through an oven, maintaining substantially the same temperature cycle as in its first trip.

In order to give the hard finish to the paper, the sandsizing coat is applied before the second oven. In the article that passes two trips through an oven, the presizing coat, clear binder and sand are on the paper on the first trip, and the two backsizing coats and the sandsizing coat are applied to the paper for its second trip through the oven.

I have discovered that gums and resins, natural or synthetic, having the desired characteristics such, for example, as rosin and other gums and resins, perform the functions desired, that is, of giving the necessary mechanical and binding strength and waterproofness which produces a binder suitable for my purposes, when employed with a suitable vehicle.

In preparing the binder for use, the gum or resin is employed in connection with a suitable vehicle which I prefer is of the type such, for example, as China-wood oil, linseed oil, or both. Certain other oils, such as semi-drying oil (soyabean oil) are suitable for that purpose.

In employing the gum or resin prepared with a vehicle such, for example, as China-wood oil, to one surface of the base or backing, I have found that the action of a vehicle involves a penetration of the base and, in some instances, this results in a weakening of the binding action and, while not sufficient to destroy the usefulness of the binder for some purposes, it is undesirable. My experience with this binderhas shown that the application of a sizing coat to the back of the sheet tends to create a suction or capillary action and in order to prevent an undue penetration, in other inventions I have employed a filler for preventing a too great penetration into the backing. However, by omitting the backsizing coats until after the binder is applied and fairly dry, I am enabled to employ a clear binder without the necessity of incorporating ingredients for preventing undue penetration. This function is further attained by the employment of a presize, that is, a sizing coat which is applied to one side of the sheet which is to receive the binder.

In my preferred practice in the production of this invention I employ presizes depending on the absorbency characteristics of the paper and the fluidity of the binder.

For dense papers, which do not absorb readily, the following examples of solutions used are representative.

Preferred Example No. 1

. Parts China-wood oil 20.5

'Ester g 2.0

Lime 0.13 Manganese oxide 0.015 Litharge 0.035 Lead acetate 0.50 Cobalt 0.015 Oleum 70.805 Turpentine 1.50 Aluminum stearate 1.000

Preferred Example No. 2

Oleum spir 96.0 Metal soaps, such as aluminum stearate, aluminum, palmitate, magnesium oleate, etc

For papers which are quite absorbent, or to control more fluid binders, a pigment may be added to such solutions as represented in Examples No. 1 and No.2. I

A representative formulafor a presizing solution containing a pigment is as follows:

Parts Formula No. 1 s 45 Formula No. 2 45 Blane fixe (magnesium silicate or the like) 10 For papers which are very absorbent I use a It is not intended that I shc Jld limit myself to the use of the above to obtain a great degree of finesse in the presizing principle.

I have further found that solutions such as cellulose acetate or cellulose acetate combined with other oils; other metal soap, such as calcium oleate, and the like; and combinations of these solutions, with pigment such as magnesium carbonate, blanc fixe, magnesium silicate, and the like; and synthetic resin solutions, such as phenol formaldehyde resins, may be used, depending on degree of control of binder penetration desired, and varying absorbency qualities of paper stock used.

One of the virtues of the presize is that it prepares the surface of the sheet to receive the clear binder and which tends to prevent undue penetration and, as well, serves to seal the front side of the sheet-in a manner similarly to the backsize, which is applied later, to seal the opposite side. I

In my preferred employment of a clear hinder the same may include the following ingredients in proportions by parts:

The sandsizing coat is applied over the grits Parts and binder and for my purposes may include the ingredients in parts as follows:

the clear binder or the hard external sand sizing coat may include the following ingredients by parts:

General Parts Vegetable drying oils 30.00 Resin 7.00 Cobalt acetate 0.25 Giisonite 1.82 Soap chips 1.49 Isinglass 1.49 Alum 1.07 Water 20.00 Oleum 36.88

Preferred Parts China-wood oi 21.00 Linseed oi 9.00 East India nubs 7.00 Cobalt acetate 0.25 Gilsonite 1.82 Soap chips 1.49 Isinglass 1.49 Alum 1.07 Water 15.00 Oleu 36.88 Ethyl alcohol 5.00

The auxiliary back sizing coat, that is, the second coat applied to the back of the sheet, may include the following ingredients by parts:

General Parts Nitrocellulose sec.) 8.8 Vegetable drying oil 13.2 Oleum 29.0 Nitrocellulose solvent 25.5 Ground silica 17.0 Resi 4.5 Water 2.0 Plasticizer or-softener.

Second back sizing solutions may include the parts as in the formula hereinafter.

Parts Nitrocellulose 9.4 Raw or heat treated. China-wood oil 10.5 Raw or blown castor oil 2.7 Ester gu 3.8 Silica 19.5 Water 1.5 Butyl cellosolv 24.0 Mineral spirit 28.6

From the foregoing, it will be understood that the pre-size may include fillers for preventing penetration where it is desired to first back size the paper, that is, apply the back sizing coats in the formulae D and E, before applying the binder.

After the pre-size, clear binder adhesive and sand have been applied to the paper, the article is suitably forced dried, that is, partially baked, as it is said; to the desired extent by subjecting it in the usual manner to a temperature which I prefer to control at approximately about 125 to 180 F., depending. upon the degree of hardness desired, and for a required period of time, preferably 36 to 48 hours. It will be obvious that the hardness of finish of the article may be varied as desired by obtaining variations in the temperature employed and the time which the article is subjected to heat.

The back sizing coats of the formulae D and E and the hard and size coat of formula C are then applied, and the article again subjected to the desired hardening temperature which may be the same as that previously pointed out.

Having thus described my invention and illustrated its use, what I claim as new and desire to secure by Letters Patent is:

1. In an abrasive article comprising a backing material, a grit or layer of abrasive particles embedded in a flexible coating, and a coating formed from a composition for increasing the coefficient of adhesion on the back surface thereof, said composition comprising a waterproofing material and a mild friction increasing material.

2. In an abrasive article comprising a backing material, and a grit or layer of abrasive particles embedded in a flexible coating and a coating formed from a composition for increasing the coefficient of adhesion on the back surface thereof in use, said composition comprising a waterproofing medium and a mild friction increasing material, the waterproofing material constituting about 60%, and the friction increasing material constituting about 40% of said composition.

3. In an abrasive article comprising a backing material, a grit or layer of abrasive particles embedded in a flexible coating, and a coating applied to the back surface of said backing for increasing the friction thereof in use, said coating applied to the back surface being formed from -a composition comprising a waterproofing base including nitro-cellulose, vegetable oil and resin.

being formed from a. composition comprising a waterproofing base including nitro-cellulose, vegetable oil and resin and a friction increasing material comprising ground silica, the waterproofing material comprising abowt 60% of the composition and the ground silica comprising about 40% of the composition.

FRANCIS G. OKIE. 

